1. Field of the Invention
The present invention relates to a method for repairing the rotor of a turbogenerator showing at least in part a tooth top cracking, said rotor bearing windings lodged in a great number of axially extending rotor slots, whereby said windings are insulated against the slot walls by so-called slot-cells, the rotor being furthermore provided with retaining rings axially held by ring keys or the like and retaining the rotor end windings against rotational forces and being shrink fitted onto the rotor ends.
2. Description of the Prior Art
Turbogenerators installed in power plants use retaining rings to contain the rotor end windings against rotational forces (FIG. 1 and FIG. 2). Thousands of the retaining rings which are currently in service were fabricated from the nonmagnetic austenitic steel 18Mn5Cr or from magnetic steel. Because this steel is susceptible to stress corrosion cracking and several failures with extended outage times have already happened, electric utilities are starting more and more to change to a new retaining ring steel 18Mn18Cr which is not susceptible to stress corrosion cracking.
Parallel to this, older power plants are being used increasingly for peak-load generation. For the turbo-generator, which was originally designed as a base load machine, this change in type of operation leads to increased fatigue loading of the rotor body and the retaining rings. These rotors have a few hundred to several thousands start/stop cycles. On several generator rotors upper sections of the rotor teeth have broken off between the tooth end and the groove of the ring key which holds the retaining ring axially (FIG. 2, FIG. 4 and FIG. 6). The teeth were broken off as a result of high fatigue load. Broken off teeth can cause serious problems in operation. This problem is called the tooth top cracking problem.
Such rotors with cracks in the rotor teeth must normally be brought back to the factory of a generator manufacturer for repair. Such a repair in general requires today also a complete rewind of the rotor. Long outage times for the power plant are the consequence. Because this repair requires also new retaining rings with different dimensions, this opportunity can also be used to replace the 18Mn5Cr retaining rings by 18Mn18Cr retaining rings. This increases additionally the reliability of a turbo-generator.
Retaining rings have to withstand different forces acting in a complicated manner. The most important forces are the centrifugal and the shrink-fit forces.
The centrifugal forces, that are generated by the ring itself and by the load of the end winding acting in the ring bore, lead to hoop stresses. Especially at 20% over-speed (test pit operation) the retaining rings are thereby highly stressed over their entire length.
The pressure of the rotor end winding (FIG. 2) in the ring bore tends to prevent any relative axial displacement of the retaining ring and end winding by developing a frictional force. Poisson contraction in the retaining ring and the thermally induced displacement of retaining ring and end winding lead therefore to axial forces in the retaining rings. Due to these forces axial stresses arise in the shrink-fit area, particularly at any transition, (see FIG. 3) where a change in bore diameter occurs. The shrink forces are greatest at standstill, and decrease with increasing speed. They also lead to hoop stresses. In addition an axial force can be produced between the usual radial shrink-fit and the ring key or the retaining ring lugs. Thus, axial stresses act also at rest.
Due to the centrifugal and shrink-fit forces hoop stresses and axial stresses are acting simultaneously especially in the shrink-fit area.
The change in stress level from standstill to full load particularly in the transition areas, represents the fatigue stress range under start/stop conditions. This fatigue loading is an important aspect to be taken into account in lifetime analysis.
Simultaneously improved manufacturing methods have become available providing an increased freedom in the selection of the ring geometry without additional costs.
As a result an optimized form of the ring and the teeth could be selected providing a considerably reduced stress level in the shrink-fit area. A drop of the fatigue stress range can thereby be achieved and accordingly the lifetime can be increased. The optimal design for the shrink-fit area on the rotor body and for the retaining ring will result from a stress analysis or lifetime analysis.
An increasing number of large new power stations, and in particular nuclear power stations, are being commissioned. Since these are being used predominantly for baseload operation, older fossil-fired power stations are being used increasingly for peak-load generation. For the turbogenerator, which was originally designed as a baseload machine, this change in type of operation leads to increased fatigue loading of the rotor body and retaining rings. In order to make an accurate condition evaluation, the cyclic stresses must be known.
A distinction is drawn between two types of cyclic operation:
Load cycling PA1 Two-shift operation PA1 removing the rotor from the stator; PA1 removing the retaining rings; PA1 cutting the slot-cells in the areas to the rotor to be machined, if necessary; PA1 covering the rotor windings with suitable cover materials and tapes and sealing the whole with parafine or the like in preparation for dry machining; PA1 dry machining the entire shrink-fit area to: thereby eliminate all cracked portions of the teeth; to obtain an optimized geometry; and machining grooves for a new retaining ring locking system; PA1 deburring the machined areas; PA1 removing all burrs; PA1 removing the sealing material and the protection covers; PA1 inserting additional U-channels between the slot walls and the remaining slot-cells for reinforcement, if necessary; PA1 locking the sandwich slot cells against axial movement; PA1 installing of new end-wedges for the slots; PA1 and finally installing new retaining rings dimensioned according to the optimized geometry of the rotor ends; and PA1 electrical tests before and after repair.
During load cycling the rotor remains at the nominal speed of rotation and the load is altered between 30% and 100% depending on the grid conditions. The alteration of the load is performed in a generator by alteration of the stator current. The stator current induces eddy current losses in the retaining rings. These eddy currents are highest in the region of the shrink-fit between the retaining ring and the rotor body, since the separation (air gap) here from the stator winding is smallest. Since the eddy current losses in the retaining ring and in the rotor body lead to heating, and this in turn to thermal stresses, load-dependent stress changes are to be expected in the region of the shrink-fit during load cycling. These load-dependent stress changes should be taken into account in a lifetime analysis, particularly if in addition an increase of generator ouput is planned.
For load changes the excitation current is also altered. Changes in the excitation current cause an alteration of the winding temperature. The related expansion of the winding generates frictional forces which lead to additional axial forces on the retaining ring. Hereby stress changes arise once more in the shrink-fit region.
If the load sinks to less than 30% of the nominal value then the turboset is generally removed from the grid and put on turning gear operation until it is next required.
This type of operation is also known as two-shift operation. The changes in rotational speed cause strong variations in the radial and axial forces on the retaining rings.
Depending on the design of the shrink-fit area larger or smaller stress variations will occur.
Normally the stress changes occuring during load cycling do not lead to any problems, provided that the coefficient of friction between the winding head and the retaining ring is sufficiently small, and provided that no very large output increases are planned for older units. But design changes where the retaining rings move further into the air gap caused by the machined tooth tops must be investigated for acceptable eddy current losses. However in two-shift operation difficulties may easily arise if the design is unsuitable. This applies particularly for older generators which were designed as baseload machines but which are to be used in future for two-shift operation.
Hence it is strongly recommended that such retaining rings and rotor bodies should be checked within the framework of a lifetime analysis.
Since, as mentioned above, retaining rings and rotor body are most highly loaded during two-shift operation, and this type of operation determines their lifetime, only the stress changes for this type of operation will be discussed in the following.
The transition areas of the shrink-fit generate higher stress levels. This effect is particularly pronounced at sharp transitions.
At the most highly stressed points cracks can be nucleated as a result of low cycle fatigue and subsequently propagate further. A further possible cause of cracking is stress corrosion. Due to the danger of crack initiation, and in particular crack propagation, the fatigue stresses must be known. The fatigue stresses must be determined for the design under consideration. Axisymmetric Finite-Element models are usually used to calculate the stress levels. The stress field in the shrink-fit area shows however a strongly 3-dimensional behaviour, due to the presence of the slots in the rotor body (see FIG. 2, FIG. 4 and FIG. 5). In order to deal with the complex stress field behaviour, 3-dimensional Finite-Element models of that area are also used. Thus, the stress levels in the retaining ring are accurately known at every rotational speed. As a consequence the inspection intervals can be prescribed and it is possible to make an estimate of defect sizes which must be sought.
Clearly, designs with sharp transitions must be investigated more closely. In particular older ring-key designs were made with sharp transition radii (FIG. 3). Cracks can propagate from these transitions. As a result it is necessary to make an exact determination of the cyclic stress taking into account all forces (in order to avoid unpleasant surprises later on). Due to differences in machining tolerances and due to the coefficients of thermal expansion the uniform destribution of the force is not always ensured. One-sided loads can lead to high stress concentrations as a result of the sharp radii.
The new designs should therefore be provided with sufficiently large transition radii (FIG. 3) so that stress concentration effects remain within reasonable limits.
For a better understanding of the invention, the following is a brief description of the cyclic loading of the rotor teeth in the shrink-fit region: The rotor teeth provide the opposing part to the retaining ring in the vicinity of the shrink-fit. In two-shift operation the rotor teeth are cyclically loaded, just as for the retaining ring.
Peak stresses at the transition areas (FIG. 3) of rotor teeth are to be minimized by appropriate design. As a result of intensively high stresses, low cycle fatigue cracks can nucleate at these points and subsequently grow until the teeth are broken off (FIG. 4).
The importance of this consideration is emphasized by several failures which have already occured. In two-shift operation rotor teeth were broken off as a result of high fatigue loads. The loads experienced by a rotor tooth are shown in FIGS. 5a and 5b). The positions of the crack nucleation are also indicated.
At standstill the shrinkage pressure p.sub.o caused compressive stresses in the notches A. At the nominal speed of rotation the shrinkage pressure p.sub.o became smaller so that the compressive stresses were also reduced. In addition a pressure p.sub.w, due to the centrifugal force of the windings, acted at the nominal speed. This effect exerted in turn tensile stresses in the notches A. In this way the stresses were altered in the tensile direction. Both loading changes, the reduction of the shrinkage pressure and the increase of the winding centrifugal force, generated a positive stress change in the notches A. This cyclic stress can be so large that within approximately 800 start/stop cycles cracks nucleated at the notches and propagated through the teeth until they broke off. By selecting a more suitable geometry for the shrink fit area it is possible to overcome this problem. FIG. 5b shows how on existing rotors the shrink fit area can be modified. Different locking systems for the retaining rings like ring keys or bayonets are possible.